FIGS. 11(a), 11(b), and 12 illustrate a double-screw kneader used as an acetylator in the production of cellulose acetate. The kneader has two rotary shafts 2 provided extending through a housing 1 thereof which defines a kneading chamber. As the rotary shafts 2 are driven by the rotating force of an electric motor 3 which is transmitted via a speed reducer 4 and groups of gears 6 provided in gear casings 5, their kneading fins 7 rotate in the housing 1 for kneading a highly viscous solution of cellulose derivative (referred to as a dope hereinafter).
Each the rotary shaft 2 is supported at both ends by two bearing units 8 between which the housing 1 is mounted. A stuffing box 11 containing a ground packing 10 is provided at each of four journal locations 9 where the two rotary shafts 2 extend through the housing 1, as shown in FIG. 13, for sealing between the housing 1 and the rotary shaft 2.
However, such a conventional sealing scheme using the ground packing 10 for sealing between the rotary shaft 2 and the housing 1 has a disadvantage that the packing 10 is easily worn out and has to be replaced by a new one at higher frequency. More specifically, the highly viscous dope kneaded in the rotary apparatus may sneak beneath the packing 10 and act as an abrasive. This causes the packing 10 to be worn out within a short duration of time. Accordingly, the packing 10 has to be frequently replaced by a new one while the operation is paused. As a result, the productivity will significantly be declined.
The packing 10 may be replaced by a mechanical seal which comprises a rotary ring mounted to the rotary shaft and a stationary ring mounted to the housing. The rings are generally fabricated of a highly wear-resistant material such as carbide alloy or ceramic. As the rotary shaft rotates, the rotary ring and the stationary ring set axially in direct contact with each other can be slid relatively at their ends thus producing a sealing effect.
However, when the mechanical seal installed in the rotary apparatus such as an acetylator also is attached by the highly viscous dope, the relative sliding movement of the rotary ring and stationary ring may be impaired. As a result, the sealing is deteriorated and stable sealing is not maintained only by a mechanical seal.
For improvement, the mechanical seal may be accompanied with a cylindrical screw sleeve which has a spiral groove provided in the outer side thereof and is mounted on the rotary shaft inwardly of the mechanical seal. As the screw sleeve is rotated together with the rotary shaft, the spiral groove drives the dope running towards the mechanical seal to return back into the housing. Such a shaft sealing apparatus made of a combination of the screw sleeve and the mechanical seal is provided for preventing the dope from reaching the mechanical seal, thus obtaining a more preferable sealing on the whole.
However, in the rotary apparatus such as a kneader for kneading a highly viscous material described above, the mounting or dismounting of the screw sleeve on the rotary shaft at the installation or maintenance process requires a significant labor. More particularly, for mounting, the screw sleeve has to be first fitted onto a corresponding end of the screw sleeve and then moved axially to the installation location on the rotary shaft where the rotary shaft penetrates through the housing. It is hence necessary at the mounting process to disassemble or dismount various gears coupled to the rotary shaft and bearing units supporting the rotary shaft. Accordingly, the mounting and dismounting of the screw sleeve will more be complicated. Also, the dismounting and mounting the gears and the bearing units will take a considerable length of time and labor, thus declining the productivity.